1. Field of the Invention
The present invention is a device for sensing and indicating the passage of a pig through a pipeline. The device is attached to a strip recorder capable of producing a stripchart which can be used in conjunction with a chart produced by an intelligent recording pig to map the interior of the pipeline in order to precisely locate defects in the pipeline.
2. Description of the Related Art
Early pipeline pigs were crude devices employed for such simple tasks as scrapping crud from the walls of a pipeline or traveling between consecutive products as the products moved through a pipeline, serving as a partition to reduce commingling of the products at their interface.
However, modern pipeline pigs contain highly sophisticated instruments and recording equipment and, therefore, are commonly referred to as "intelligent" pigs. These intelligent pigs are able to sense and record internal features of the pipeline which they encounter as they travel through the pipeline. These features include such things as welds which connect the pipeline segments together to form the pipeline and anomalies, such as dents or damaged places in the pipeline.
One goal in using the information obtained by the intelligent pig is to link a physical benchmark location, selected on the soil surface above the pipeline, with a point in the data recorded by the pig. When this linkage can be made, workmen can dig at the precise location to reach the point within the pipeline requiring attention, thus, saving time and money in unnecessary digging and searching. Another goal is to be able to monitor the pig as it moves through the pipeline in order to determine whether the pig is approaching or traveling away from a particular point. This information is useful in various situations, but particularly useful when the pig must be retrieved from the pipeline at a particular point on the pipeline and crews must be notified prior to the arrival of the pig at that point so that they will be available to retrieve the pig.
Magnetometers have been made to detect passage of a pig as shown, for example, by U.S. Pat. No. 4,714,888 issued on Dec. 22, 1987 to French et al. Use of a magnetometer provides one piece of information, the time of pig passage, that can be used to link a point in the pig data file to the surface benchmark location where the magnetometer is located. The pig passage time will be recorded on the pig data file and recorded on a stripchart attached to the magnetometer at the soil surface. In order to use these recorded pig passage times, the recorded times must be corrected for drift between the time source used at the surface and the time source used on the pig. This is typically done by recording the time the pig is launched and the time the pig is received for both time sources and then applying a linear correction factor to one of the time sources or references. However, the assumption of linear time drift is probably not accurate since most of the time drift is caused by temperature. The drift due to temperature can be reduced by using an oven controlled time source or by using a temperature compensated time source. However, if the pig run is longer than a few days, the time drift errors can be difficult to correct.
Once the time drift errors are addressed and a time of passage past the benchmark location is arrived at, this time can be referenced to an event or feature recorded by the pig. Intelligent instrumented pigs can either continuously record data, like a tape recorder or intermittently record data. The intermittent data recording technique typically records data when important information is gathered or on a periodic basis for various reasons. For pigs that continuously record data, the time of pig passage can be linked to an exact point in the pig's data record. For pigs that record data sporadically, an event recorded slightly before and an event recorded slightly after the passage of the pig must be found. The time of pig passage will be linked to the pig data by assuming an average pig velocity between the two events and then calculating a distance offset from one of the events based on the time difference between the event and the pig passage time. In general, the accuracy of the benchmark position in the pig data file is directly related to the time gap between the two events and the speed of the pig.
Geophones have been used to track and/or to detect passage of a pig as shown, for example, by U.S. Pat. No. 4,590,799 issued May 27, 1987 to Brown et al. Typically, geophones are used to listen to the sounds made by the pig as the pig moves in the pipeline. Most pigs make a noise as their cups cross a weld between pipe joints. The pig speed can be estimated by assuming pipe joints are 40 feet long, and by listening for the pig to cross the welds and counting the time required for the pig to cross several welds. Geophones are also used to detect pig passage by listening to the sound of the pig as the sound gets louder and then softer as the pig passes the listening location. The exact moment the pig is under the listening location is approximately the point at which the noise is the loudest.
By employing two geophones equally spaced apart around a benchmark location or by using a magnetometer and geophone combination, the present invention produces a benchmark system that performs much better than the simple sum of the individual parts, i.e. the magnetometer and the geophone benchmark system. If we simply added the magnetometer and geophone systems together we would have a system that accurately pinpoints the moment of pig passage (magnetometer) and also verifies that the pig is moving and has passed at an approximate time (geophone).
By employing a magnetometer in conjunction with the geophone system, the present invention records not only the time of the pig passage from the magnetometer signal, but also records the time of dozens of welds from the geophone signals on both sides of the pig passage time. Although the human ear can sometimes distinguish the sound of each pig cup crossing a weld (for pigs provided with a cup at each end of the pig), accurately timing these two (or more when more than two cups are provided on the pig) sounds would be almost impossible. The electronics in the present invention can be made sensitive and accurate enough to record the time each cup crosses a weld. This means that the electronics can record the time that the pig was within a few inches of a position (near a weld) in a pipeline. Most modern instrumented pigs record time and distance at least at each pipeline joint weld. The series of weld times recorded by the present invention are a virtual fingerprint of the pipeline since the sequence of pipe segment lengths are probably unique within the entire pipeline. The highly accurate time of pig passage below the magnetometer is used to give an approximate time to check for a match between the sequence of weld times recorded by the geophones and to determine the position of the benchmark location between adjacent welds. The weld times in the data recorded by the pig are aligned with the weld times recorded by the present invention. The time correction factor between the two time sources can be calculated. The time of pig passage is then linked with the data recorded by the pig.
One unexpected result of combining a magnetometer and a geophone is that the accuracy of the benchmark link to the recorded data is not directly related to the accuracy and stability of the time sources. This means the invention will work with easily obtainable time sources and work for pig runs of longer than a few days without loss of accuracy. Also, the benchmark location correlation between the present invention and the pig is better than any other benchmark technique now in use. The present invention can be made into a small hand carried, battery powered unit that is easy to use. And since only a magnet and time recording means are required in the instrumented pig, this invention can be used with virtually all instrumented pigs. Also because the signal received by the magnetometer is not dependent on pig speed, the magnetometer can detect pig passage very accurately and can detect pig passage even if the pipeline is buried much deeper than the normal three feet below the soil surface.